The present invention pertains to improvements in the field of catalytic materials. More particularly, the invention relates to a mesoporous zeolitic material having microporous crystalline mesopore walls and to a process of preparing same.
Zeolites and related crystalline molecular sieves are widely used as catalysts in the industry since they possess catalytically active sites as well as uniformly sized and shaped micropores, that allow for their use as shaped selective catalysts in, for instance, oil refining, petrochemistry and organic synthesis. However, due to the pore size constraints, the unique catalytic properties of zeolites are limited to reactant molecules having kinetic diameters below 10 xc3x85.
Recently, a group of researchers at Mobil Co. reported a series of mesoporous molecular sieves, named M41 S series, in U.S. Pat. Nos. 5,057,296 and 5,102,643. These molecular sieves with mesopore diameters of 15-100 xc3x85 overcome the limitation of microporous zeolites and allow the diffusion of larger molecules. These materials, however, are amorphous solids. Amorphous silica-aluminas have much weaker acid sites than zeolites and thus do not exhibit the spectacular catalytic properties of acidic zeolites. Moreover, their hydrothermal stability is low and, as a consequence, their industrial use as catalysts is very limited.
It is therefore an object of the present invention to provide a new type of catalytic material having a bimodal pore structure, which overcomes the limitations of both zeolites and mesoporous molecular sieves.
According to one aspect of the invention, there is provided a mesoporous zeolitic material having a stereoregular arrangement of uniformly-sized mesopores with diameters ranging from 20 to 500 xc3x85 and walls having a microporous nanocrystalline structure. The mesopore walls have a stereoregular arrangement of uniformly-sized micropores with diameters less than 15 xc3x85.
The present invention also provides, in another aspect thereof, a method of preparing a mesoporous zeolitic material as defined above. The method according to the invention comprises the steps of:
a) providing a mesoporous silica having a stereoregular arrangement of uniformly-sized mesopores having diameters ranging from 20 to 500 xc3x85 and walls having an amorphous structure;
b) impregnating the mesoporous silica with a zeolite-templating compound;
c) subjecting the impregnated mesoporous silica obtained in step (b) to a heat treatment at a temperature and for a period of time sufficient to cause transformation of the amorphous structure into a microporous nanocrystalline structure, thereby obtaining a mesoporous zeolitic material with mesopore walls having a stereoregular arrangement of uniformly-sized micropores with diameters less than 15 xc3x85; and
d) removing the zeolite-templating compound from the mesoporous zeolitic material obtained in step (c).
The expression xe2x80x9cnanocrystalline structurexe2x80x9d as used herein refers to a structure comprising crystals with sizes of the order of 10 nanometers or less.